**1. Introduction**

At present, heat-resistant nickel alloys such as Inconel 718 type are widely used to manufacture critical parts operating under high thermal loads. For example, it can be nozzles and working turbine blades, fairings, et cetera. The high-performance characteristics of parts made of nickel alloys determine the difficulties in their machining, accompanied by increased heat and power loads on the cutting tool [1–3]. The machinability factor for high-temperature nickel alloys is in the order of 0.25 compared to the machining of C45 steel (according to EN 10083-2: 2006). Simultaneously, solid carbide end mills are the most popular and versatile tool for machining aircraft parts. A modern approach to solving the problem of increasing machining nickel alloys' productivity over the past years is the development and use for these purposes of end mills made of tool ceramics [4–6]. Today, large international companies, which are recognized leaders in cutting tools production (Kennametal, Mitsubishi Materials, Iscar, and others), produce solid ceramic end mills on an industrial scale [7–9]. Figure 1a shows an example of industrial use of solid ceramic end mills made of SiAlON material for high-speed milling a turbine blade made of nickel alloy on a multi-axis computer numerical control (CNC) machine.

The main advantage of using ceramic end mills when machining nickel alloys over carbide tools is higher heat resistance. Nickel alloys of the Inconel type begin to soften at cutting temperatures of 800 ◦C and above (Figure 1b), after which significantly lower

**Citation:** Grigoriev, S.N.; Volosova, M.A.; Fedorov, S.V.; Okunkova, A.A.; Pivkin, P.M.; Peretyagin, P.Y.; Ershov, A. Development of DLC-Coated Solid SiAlON/TiN Ceramic End Mills for Nickel Alloy Machining: Problems and Prospects. *Coatings* **2021**, *11*, 532. https://doi.org/10.3390/ coatings11050532

Received: 13 April 2021 Accepted: 28 April 2021 Published: 29 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

power loads accompany their machining on the tool [1]. These temperatures correspond to cutting speeds above 350 m/min. Tool ceramics can be successfully operated due to their higher heat resistance in this high-speed milling mode. Sintered hard alloys at cutting temperatures over 800 ◦C lose their hardness rapidly (Figure 1b), which excludes the possibility of their use in high-speed milling nickel alloys using the surface layer plasticization effect [10,11].

**Figure 1.** An example of industrial use of SiAlON solid ceramic mills in high-speed milling of a nickel alloy turbine blade (**a**) and the variation in surface hardness of end mills produced from ceramics and hard alloy (**b**) as a function of the cutting temperature.

Practice shows that among a wide variety of well-known technical ceramics brands for tool purposes for the solid ceramic end mills manufacturing, the most suitable ceramic material is SiAlON (silicon-aluminum oxynitride), which belongs to the class of ceramics based on silicon nitrides [12–15]. This ceramic consists of three or more phases: α-sialonic, β-sialonic, and amorphous or partially crystallized grain-boundary phases. Sintered ceramics based on α/β-sialons are characterized by a unique combination of even higher hardness than traditional silicon nitride while providing a high level of strength properties. The α-sialon phase has a high hardness that is retained at elevated temperatures, and the βsialon phase has a high impact strength and fracture toughness. α- and β-sialons are ideally combined, and the ratio between these phases can be quite easily varied when preparing powder compositions (raw materials or precursors) for subsequent sintering [4–6]. It makes it possible to obtain a different set of physical and mechanical properties of the sintered ceramic necessary for the cutting tool's specific operating conditions [16–19].

Moreover, any ceramics, even those obtained using the most advanced technology, are structurally inhomogeneous materials that a priori contain certain defects (such as micropores). The sintering process of ceramic powders involves providing an increased temperature mode and is very technologically complex. In sintering, grain growth can occur; a significant amount of residual glassy phase can be formed, which worsens the hardness and strength of the material during high-temperature operation [20,21]. Excessive porosity can form at a low rate of grain-boundary diffusion and insufficient shrinkage of the powder composition, which sharply worsens the sintered ceramics' fracture toughness [22–24]. It should be borne in mind that a sintered ceramic workpiece, to obtain multi-edge tools, must be subjected to dimensional shaping processing by mechanical methods. One of the main machining methods is grinding with diamond abrasive tools by CNC grinding and sharpening machines. Grinding is also extremely difficult and energyintensive. Additional difficulties are associated with the reduced electrical and thermal conductivity of ceramics. The physical and technical nature of the diamond grinding process is such that it introduces additional defects (chips, microcracks) into the surface layer of a ceramic workpiece [22,25–27]. Therefore, if a workpiece containing cracks and pores was formed at the sintering stage, they will inevitably become local foci of microfracture

and chips of the cutting part, which will reduce the tool performance. That is why leading researchers and technologists prioritize developments in improving technological processes for sintering ceramics and improving the ceramic powder composition to improve the basic physical and mechanical properties of ceramic blanks with a simultaneous improvement of their machinability.

A promising technological process for sintering SiAlON ceramics is the spark plasma sintering (SPS) method. The sintered powder composition and the used mold are heated by passing high-frequency low-voltage pulses of direct electric current through them. Compared to traditional hot pressing, SPS can significantly reduce the holding time at the maximum temperature (in the order of several minutes). Due to the direct transmission of electric current, it is possible to reduce grain growth rate since it allows high heating and cooling rates of the ceramics [28–30].

Large reserves for improving the physical and mechanical properties of ceramics based on SiAlON are introducing alloying components. For example, the introduction of up to 20 wt.% TiN nanoparticles into a powder composition based on α/β-sialons can provide an optimal ratio between hardness and impact toughness, improve electrical and thermal conductivity, crack resistance, and machinability [31–36]. Besides, stabilization during unstable high-temperature phases sintering, which is carried out by alloying with certain stabilizing elements based on rare-earth metal oxides, is of great importance. Today, Nd, Sm, Gd, Dy, Y, and Yb elements introduced in the form of nanoparticles into a powder composition with a volume of up to 7 wt.% are sufficiently studied and have proven a certain efficiency in SiAlON production [37–40]. World manufacturers, currently industrializing solid ceramic end mills, sinter powder compositions based on α/β-sialons with stabilizing additives Y2O3 and Yb2O3 (the latter option is recognized as more effective). The introduction of alloying rare-earth metals yttrium and ytterbium complicates powder composition preparation and sintering and increases the cost of an already expensive end product with apparent positive aspects. For example, the average selling price per unit for a one-piece ceramic four-flute endmill with a diameter of 10 mm is over 500 Euro. Such a high cost is partly offset by a manifold increase in productivity and durability during operation. Simultaneously, such a tool, compared to hard alloy, is not suitable for subsequent regrinding after reaching the limit wear by the cutting part. This constrains the broader distribution of solid ceramic end mills in mechanical engineering. Today, such a tool is not publicly available for manufacturing enterprises, and exploratory research aimed at finding approaches to simplifying solid ceramic end mills manufacturing technology and reducing their manufacturing costs is relevant.

One of the approaches to increasing the wear resistance of one-piece ceramic end mills can be various coatings deposition on their working surfaces based on complex nitrides and diamond-like carbon structures. This approach is based on the fact that under various loading conditions of the tool cutting part, its surface layer is the most loaded, and in many respects, this layer determines the wear resistance under the influence of external loads [41–44]. The coatings' effectiveness in increasing the wear resistance of solid ceramic mills in machining nickel alloys has not been experimentally studied until now. Simultaneously, in assessing the effectiveness of this approach to improve assembled turning tools and end mills equipped with ceramic plates, researchers do not have one single point of view [25,26]. Some experts are not inclined to consider coatings' deposition as a viable approach to improve ceramic tools' wear resistance. It can be assumed that such conclusions are the result of overestimated expectations from coatings applied to ceramics. One should not expect an effect comparable to that achieved for carbide tools (multiple increases in durability) from coatings. Initially, it is necessary to proceed from the fact that the role of coatings deposited onto ceramics is specific, and even a twofold increase in the ceramics' resistance should be considered a good result. Many experimental works demonstrate that with rationally chosen deposition technology, architecture, and coatings' composition, their particular possibilities for increasing resistance are quite real and significant [42–46]. For example, the authors of this work have achieved an increase in

resistance during turning and milling hardened steels by 1.4–1.9 times in previous studies when vacuum-plasma coatings' deposition onto cutting inserts made of tool ceramics based on Al2O3 + TiC and Al2O3 + SiC [25,26,42], which was a consequence of the effect of coatings on the properties of the surface and the surface layer of ceramic plates. The changes were manifested in the form of a decrease in the friction coefficient between the tool and the processed material, some improvement in strength characteristics, smoothing and reduction of the height of surface microroughness, and a decrease ("healing") of ceramic plates' surface defects that were formed during diamond grinding [25,47]. Thus, there are prerequisites for the fact that the approach to increasing the wear resistance of solid ceramic mills, based on various coatings' deposition onto their working surfaces that can also have a positive effect.

Within the framework of this work, a set of problems was solved, which can be formulated as follows:

